The present invention relates to thin hydrogen-selective palladium-alloy membranes and the like and to novel methods of preparing the same; being more particularly directed to such alloys produced by novel solid-solid interdiffusion of pressure-contacting films or layers of palladium metal with palladium-enhancing metals, such as copper and silver and the like.
This application is based in part upon the discovery disclosed in our U.S. patent application Ser. No. 880,990, now U.S. Pat. No. 5,904,754 in which an assembly of a palladium/copper (60%:40% by weight) alloy foil is edge-area diffusion-bonded to a copper-surfaced metallic frame. The advantages of such hydrogen-selective Pd/Cu alloy membranes have been cited in this application which is incorporated herein by reference.
Commonly, thin 60% Pd/40% Cu foil membranes are made from billets which are solidified melts of the two metals in uniform admixture. Starting with the billet, thinning involves an elaborate and expensive sequence of steps of rolling and annealing, with the cost increasing disproportionately as the practical low thickness limit of about 0.001 inch of a pinhole-free foil is reached. Similarly, other hydrogen-selective palladium alloy foils are generally made from alloy billets. These include in particular the well known silver/palladium alloy containing 23-25% silver (herein Pd/23-25% Ag). In general, to form a suitable palladium bearing membrane, palladium is alloyed with one (and sometimes more than one) stability- and/or permeability-enhancing metal, as shown, for example, in the publication entitled "Hyperpure Hydrogen From Palladium Alloy Membrane Permeation", by R. Goto, Chemical Economy and Engineering Review (Japan), 2(10), pp 44-50 (1970), also included herein by reference.
We have now found that the edge-area intermetallic diffusion-bonding technique of said copending application can be extended to the generation of, preferably, thin palladium/copper and the like selected alloy foils, including the above-referred silver/palladium alloy. The method is especially advantageous in producing less than about 25 micrometer (ca. 0.001") thick membranes. Here the thinner the better for intermetallic diffusion, in contrast to the limitation of excessive costs of thin sheet rolling from billets.
The prior art in this field has been oriented toward determining intermetallic diffusion constants for Pd/Cu and Pd/Ag binary, thin film systems. Intermetallic diffusion of Pd/Ag layers has been discussed in "Interdiffusion studies in silver/palladium couples by means of Auger depth profiling", by Bukaluk and Rozwadowski in Vacuum 46(5/6), pp 579-582 (1995) and in "Interdiffusivities in silver-palladium composition-modulated foils", by Henein and Hilliard in Journal of Applied Physics 55(8), pp 2895-2900 (1984). The Pd/Cu system has been likewise investigated in "Diffusion and Size Effect in Thin Films of the Systems Cu-Pd and Cu-Ag", by Grebennik and Zyman in Fiz. metal metalloved. (Russian), 32(4), pp 891-893 (1971), and in "Study of interdiffusion in Pd/Cu multilayered films by Auger depth profiling", by Jeon, et. al., Journal of Applied Physics, 75(12), pp 7825-7828 (1994). While these studies attempt to understand the fundamentals of the intermetallic diffusion process they do not disclose methods for fabricating thin foils for use as hydrogen separation membranes.
In accordance with the present invention, an improved technique and method of fabricating hydrogen-selective alloy membranes by a novel intermetallic solid-metal interdiffusion process is now provided, producing also improved stability and/or enhanced hydrogen permeability in the resulting alloy membranes.